Added library-level comments for polymer and polymer_expressions

pkg/polymer_expressions/lib/polymer_expressions.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+/**
+ * A syntax used with polymer elements that
+ * allows for complex binding expressions, including
+ * property access, function invocation,
+ * list/map indexing, and two-way filtering.
+ *
+ * When you install polymer.dart,
+ * polymer_expressions is automatically installed as well.
+ *
+ * Polymer expressions are part of the Polymer.dart project.
+ * Refer to the
+ * [Polymer.dart](http://www.dartlang.org/polymer-dart/)
+ * home page for example code, project status, and
+ * information about how to get started using Polymer.dart in your apps.
+ * 
+ * ## Other resources
+ *
+ * Learn more about the polymer_expressions package
+ * at the pub repository:
+ * [polymer_expressions](http://pub.dartlang.org/packages/polymer_expressions).
+ *
+ */
+
 library polymer_expressions;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:observe/observe.dart';
 
 import 'eval.dart';
 import 'expression.dart';
 import 'parser.dart';
@@ skipping 85 matching lines @@
 
   getValueWorkaround(key) {
     if (key == _VALUE) return value;
   }
 
   setValueWorkaround(key, v) {
     if (key == _VALUE) value = v;
   }
 
 }